KAIST "Korea-US Joint Research Team Develops 'Ultrathin Photodiode'"

A Korea-US joint research team has developed next-generation high-resolution image sensor technology. The developed technology offers higher power efficiency compared to existing sensors and enables the implementation of high-performance image sensors with smaller sizes. In particular, the joint research team anticipates a high possibility of market entry in the future by securing the core technology of ultra-high-resolution short-wave infrared (SWIR) image sensor technology, which has been led by the Japanese company Sony.

KAIST announced on the 20th that Professor Sanghyun Kim’s team from the Department of Electrical Engineering collaborated with Inha University and Yale University in the US to develop an ultra-thin broadband photodiode (PD).

(From left) Professor Sanghyun Kim, Department of Electrical Engineering, KAIST; Professor Daemyeong Geum, Inha University; Postdoctoral Researcher Jinha Lim, Yale University. Courtesy of KAIST

First, the joint research team improved the trade-off between the absorption layer thickness and quantum efficiency seen in existing photodiode technology, achieving a high quantum efficiency of over 70% even with an absorption layer thinner than 1 μm.

When the absorption layer becomes thinner, the pixel process is simplified, enabling high resolution. Additionally, carrier diffusion becomes smoother, which is advantageous for acquiring photocarriers, and it also offers cost reduction benefits.

However, generally, thinning the absorption layer reduces the absorption of long-wavelength light. The joint research team succeeded in reducing the absorption layer thickness of existing technology by about 70% while overcoming this issue.

The joint research team also demonstrated that by introducing a guided-mode resonance (GMR) structure?a concept used in electromagnetics where specific waves (light) resonate (forming strong electric and magnetic fields) at certain wavelengths?it is possible to maintain high-efficiency light absorption across a broad spectrum from 400 nm to 1700 nm.

This wavelength range includes not only the visible light region but also the short-wave infrared (SWIR) region, which is expected to play an important role in various industrial applications.

Performance improvement in the short-wave infrared region can be a new turning point in the development of next-generation image sensors. In particular, the guided-mode resonance structure has the potential to enhance resolution and other performance aspects through hybrid integration with complementary metal-oxide-semiconductor (CMOS)-based readout integrated circuits (ROIC) and monolithic 3D integration.

The joint research team expects that low-power devices and ultra-high-resolution imaging technology will be widely utilized in future ultra-high-resolution image sensor fields, ranging from digital cameras, security systems, medical and industrial image sensor applications to autonomous driving in automobiles, aviation, and satellite observation.

Professor Sanghyun Kim of KAIST said, "The joint research team has demonstrated that much higher performance than existing technology can be achieved even with an ultra-thin absorption layer. In particular, by securing the core technology for ultra-high-resolution short-wave infrared (SWIR) image sensor technology led by Sony in the global market, we expect possible market entry in the future."

Meanwhile, this research was conducted with the support of the National Research Foundation of Korea. The research paper lists Professor Daemyung Geum of Inha University (former KAIST postdoctoral researcher) and Dr. Jinha Lim (current postdoctoral researcher at Yale University) as co-first authors.